Aura: A mission dedicated to the health of Earth's atmosphere


On July 15, 2004 at 3:02 a.m., NASA launched the Aura satellite, the third flagship in a series of Earth-observing satellites designed to view Earth as a whole system, observe the net results of complex interactions within the climate system, and understand how the planet is changing in response to natural and human influences. Aura was exclusively designed to study the composition, chemistry, and dynamics of the Earth’s upper and lower atmosphere by employing four instruments on a single platform. Each instrument provides unique and complementary capabilities that will enable daily global observations of Earth’s atmospheric ozone layer, air quality, and key climate parameters.

Aura SatelliteThis artist’s rendering shows the Aura satellite in its orbit roughly 705 km above the Earth just east of the North Carolina coastline. (This image is also available in a higher resolution.) (Image courtesy of Jesse Allen, NASA Earth Observatory/SSAI).

Over its six-year life span, Aura will provide high-quality data to help answer these important questions regarding the health of Earth’s atmosphere:

  • Is the stratospheric ozone layer recovering?
  • What are the processes controlling air quality?
  • How is the Earth’s climate changing?

next: Is the stratospheric ozone layer recovering?


by Steve Graham
December 7, 2004



Aura: A mission dedicated to the health of Earth's atmosphere


Is the stratospheric ozone layer recovering?

The stratospheric ozone layer shields life on Earth from harmful ultraviolet (UV) solar radiation (wavelengths shorter than 340 nm). Research shows that excess exposure to the Sun’s UV radiation is harmful to plants, including crops, and causes skin cancer and eye problems in humans and animals. Excess UV radiation exposure may also suppress the human immune system.

Ozone forms naturally in the stratosphere as incoming solar UV radiation hits oxygen molecules (O2) and breaks them apart into free-floating individual oxygen atoms. These oxygen atoms can then combine with O2 molecules to form ozone molecules (O3). Ozone is destroyed when an ozone molecule combines with an oxygen atom to form two oxygen molecules; or through certain chemical reactions involving molecules containing hydrogen, nitrogen, chlorine, or bromine atoms. (For more details on how stratospheric ozone is formed and destroyed, please see NASA’s Ozone fact sheet.

The atmosphere maintains a natural balance between ozone formation and destruction. But the natural balance of chemicals in the stratosphere has changed over the last three decades, particularly due to the presence of man-made chlorofluorocarbons (CFCs). CFCs are produced by chemical industries for use as refrigerants, solvents, and propellants. Because CFCs are non-reactive they tend to build up in the atmosphere and are eventually destroyed high in the stratosphere where they are no longer shielded by the ozone layer from UV radiation.

Destruction of CFC molecules yields free-floating chlorine atoms, which play an active role in destroying ozone molecules. Other man-made gases, such as nitrous oxide (N2O) and bromine compounds, are also broken down in the stratosphere and play active roles in ozone destruction.



Stratospheric ozone processes

Stratospheric ozone processes The stratospheric ozone layer shields life on Earth from the Sun’s harmful ultraviolet radiation. Chemicals that destroy ozone are formed by industrial and natural processes. With the exception of volcanic injection and aircraft exhaust, these chemicals are carried up into the stratosphere by strong upward-moving air currents in the tropics. Methane (CH4), chlorofluorocarbons (CFCs), nitrous oxide (N2O) and water are injected into the stratosphere through towering tropical cumulus clouds. These compounds are broken down by the ultraviolet radiation in the stratosphere. Byproducts of the breakdown of these chemicals form “radicals”—such as nitrogen dioxide (NO2) and chlorine monoxide (ClO)—that play an active role in ozone destruction. Aerosols and clouds can accelerate ozone loss through reactions on cloud surfaces. Thus, volcanic clouds and polar stratospheric clouds can indirectly contribute to ozone loss. This illustration is also available at a higher resolution.  (Illustration courtesy Barbara Summey, SSAI)


Satellite observations of the ozone layer began in the 1970s when the possibility of ozone depletion was just becoming an environmental concern. NASA’s Total Ozone Mapping Spectrometer (TOMS) and Stratospheric Aerosol and Gas Experiment (SAGE) have provided long-term records of ozone. In 1985 the British Antarctic Survey reported an unexpectedly deep ozone depletion over Antarctica. The annual occurrence of this depletion, popularly known as the ozone hole, alarmed scientists. Specially equipped high-altitude NASA aircraft established that the ozone hole was due to man-made chlorine. TOMS and SAGE data also showed smaller but significant ozone losses outside the Antarctic region. In 1987 an international agreement known as the Montreal Protocol restricted CFC production. In 1992, the Copenhagen amendments to the Montreal Protocol set a schedule to eliminate all production of CFCs.

Severe ozone depletion occurs in winter and spring over both polar regions. The polar stratosphere becomes very cold in winter because of the absence of sunlight and because strong winds isolate the polar air. Stratospheric temperatures fall below –88°C. Polar stratospheric clouds (PSCs) form at these low temperatures. The reservoir gases HCl and ClONO2 react on the surfaces of cloud particles and release chlorine.

  Ozone hole globes

These TOMS images illustrate the development of the ozone hole during the 1980s and 1990s. Dark blue colors correspond to the thinnest ozone, while light blue, green, and yellow pixels indicate progressively thicker ozone. (Image courtesy of the NASA GSFC Scientific Visualization Studio.)

  Picture of polar stratospheric clouds

Ground-based data have shown that CFC amounts in the troposphere are leveling off, while data from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) show that amounts of HCl, a chlorine reservoir that is produced when CFCs are broken apart, are leveling off as well (See Global HCl figure). Recent studies show that the rate of ozone depletion is also decreasing.


Thin clouds made of ice, nitric acid, and sulfuric acid mixtures form in the polar stratosphere when temperatures drop below -88°C (-126°F). In such polar stratospheric clouds (PSCs) active forms of chlorine are released from their reservoirs. This particular PSC appeared over Iceland at an altitude of 22 km on February 4, 2003. Its beautiful colors result from diffraction of sunlight by ice particles. (Photo courtesy Mark Schoeberl, NASA GSFC).

Recovery of the ozone layer may not be as simple as eliminating the manufacture of CFCs. Climate change will alter ozone recovery because greenhouse gas increases will cause the stratosphere to cool. This cooling may temporarily slow the recovery of the ozone layer in the polar regions, but will accelerate ozone recovery at low and middle latitudes.

What will Aura do?

Aura’s instruments will observe the important sources, radicals, and reservoir gases active in ozone chemistry. Aura data will improve our ability to predict ozone change. Aura data will also help untangle the roles of transport and chemistry in determining ozone trends. For details about how Aura's individual instruments contribute to understanding stratospheric ozone, please see the "Aura's Instruments" section of this Fact Sheet.

next: What are the processes controlling air quality?
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UARS HALOE measurements of stratospheric chlorine (HCl) at 55 km show that international controls on CFCs are working. HCl amounts have leveled off and are now decreasing. Aura's MLS will continue the HALOE HCl record.


Aura: A mission dedicated to the health of Earth's atmosphere


What are the processes controlling air quality?

Agriculture and industrial activity have grown dramatically along with the human population. Consequently, in parts of the world, increased emissions of pollutants have significantly degraded air quality. Respiratory problems and even premature death due to air pollution occur in urban and some rural areas of both the industrialized and developing countries. Wide-spread burning for agricultural purposes (biomass burning) and forest fires also contribute to poor air quality, particularly in the tropics. The list of culprits in the degradation of air quality includes tropospheric ozone, which is a toxic gas, and the chemicals that form ozone. These ozone precursors are nitrogen oxides, carbon monoxide, methane, and other hydrocarbons. Human activities such as biomass burning, inefficient coal combustion, other industrial activities, and vehicular traffic all produce ozone precursors.

The U.S. Environmental Protection Agency (EPA) has identified six criteria pollutants: carbon monoxide, nitrogen dioxide, sulfur dioxide, ozone, lead, and particulates (aerosols). Of these six pollutants, ozone has proved the most difficult to control. Ozone chemistry is complex, making it difficult to quantify the contributions to poor local air quality. Pollutant emission inventories needed for predicting air quality are uncertain by as much as 50 percent. Also uncertain is the amount of ozone that enters the troposphere from the stratosphere.

For local governments struggling to meet national air quality standards, knowing more about the sources and transport of air pollutants has become an important issue. Most pollution sources are local but satellite observations show that winds can carry pollutants for great distances, for example from the western and mid-western states to the east coast of the United States, and sometimes even from one continent to another. Observations and models show that pollutants from Southeast Asia contribute to poor air quality in India. Pollutants crossing from China to Japan reach the west coast of the United States. Pollutants originating in the United States can reduce air quality in Europe. Precursor gases for as much as ten percent of ozone in surface air in the United States may originate outside the country. We have yet to quantify the extent of inter-regional and inter-continental pollution transport.




The sources of tropospheric ozoneTropospheric ozone comes from several sources. Biomass burning and industrial activity produce carbon monoxide (CO) and volatile organic compounds (VOCs) which are oxidized to form ozone. Nitrogen oxides (NOx) from industrial processes, biomass burning, automobile exhaust and lightning also form tropospheric ozone. A small amount of tropospheric ozone also comes from the stratospheric ozone layer. This illustration is also available at a higher resolution. (Image courtesy of Barbara Summey, SSAI.)


Long-Range Pollution Transport

The atmosphere can transport pollutants long distances from their source. Satellite measurements by EOS Terra’s Measurements of Pollution in the Troposphere (MOPITT) instrument have shown carbon monoxide streams extending almost 18,000 km from their source. NASA's Total Ozone Mapping Spectrometer (TOMS) has tracked dust and smoke events from Northern China to the east coast of the United States.

On July 7, 2002, the Moderate Resolution Imaging Spectroradiometer (MODIS) on EOS Terra and TOMS captured smoke from Canadian forest fires as the winds transported it southward. This pollution event was responsible for elevated surface ozone levels along the east coast. TOMS has high sensitivity to aerosols like smoke and dust when they are elevated above the surface layers. The Ozone Measuring Instrument (OMI) on Aura will make similar measurements with better spatial resolution and will provide new information about aerosol characteristics.

MODIS image of smoke over the eastern U.S. MODIS July 7, 2002 image shows smoke streaming southward from forest fires in Canda.

TOMS image of aerosols from smoke over the eastern U.S. TOMS July 7, 2002 image shows the aerosol index of smoke streaming southward from forest fires in Canada.

What will Aura do?

The Aura instruments are designed to study tropospheric chemistry. Together Aura’s instruments provide global monitoring of air pollution on a daily basis. They measure five of the six EPA criteria pollutants (all except lead). Aura will provide data of suitable accuracy to improve industrial emission inventories, and also to help distinguish between industrial and natural sources. Because of Aura, we will be able to improve air quality forecast models. For details about how Aura's individual instruments contribute to understanding air quality, please see the "Aura's Instruments" section of this Fact Sheet.

next: How is the Earth's Climate Changing?
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Aura: A mission dedicated to the health of Earth's atmosphere


How is the Earth's climate changing?

Water vapor, carbon dioxide and other greenhouse gases trap infrared radiation that would otherwise escape to space. This phenomenon, the greenhouse effect, makes the Earth habitable. Increased atmospheric emissions from industrial and agricultural activities are causing climate change. Industry and agriculture produce trace gases that trap infrared radiation. The concentrations of many of these gases have increased and thus have added to the greenhouse effect. Since the turn of the century, the global mean lower tropospheric temperature has increased by more than 0.4°C. This increase has been greater than during any other century in the last 1,000 years.

Ozone plays multiple roles in climate change because it absorbs both ultraviolet radiation from the Sun and infrared radiation from the Earth’s surface. Tropospheric ozone is as important as methane as a greenhouse gas contributor to climate change. An accurate measurement of the distribution of tropospheric ozone will improve climate modeling and climate predictions.

Aerosols are an important but uncertain agent of climate change. Aerosols alter atmospheric temperatures by absorbing and scattering radiation. Aerosols can both warm or cool the troposphere, depending on their type and location. Therefore, aerosols also modify clouds and affect precipitation. Sulfate aerosols can reduce cloud droplet size, making clouds brighter so that they reflect more solar energy. Black carbon aerosols strongly absorb solar radiation, warming the mid-troposphere and reducing cloud formation. Poor knowledge of the global distribution of aerosols contributes to a large uncertainty in climate prediction.

Ozone absorbs solar radiation, warming the stratosphere. Man-made chlorofluorocarbons have caused ozone depletion, leading to lower temperatures. Low temperatures, in turn, lead to more persistent polar stratospheric clouds and cause further ozone depletion in polar regions.

Increasing carbon dioxide also affects the climate of the upper atmosphere. Where the atmosphere is thin, increasing CO2 emits more radiation to space, thus cooling the stratosphere. Observations show that over recent decades, the mid to upper stratosphere has cooled by 1 to 6°C (2 to 11°F) primarily due to increases in CO2. This cooling will produce circulation changes in the stratosphere that will change how trace gases are transported.

Water vaopr map from MLSWater vapor map from MLSColorbar for water vapor mapWater vapor measurements from the Microwave Limb Sounder (MLS) onboard the Upper Atmospheric Research Satellite (UARS) show the contrast between 1996 (a "normal" year) and 1997 (an "El Niño" year). In 1996 the most convection and the highest mixing ratios for tropical upper tropospheric water vapor (red) occur over Indonesia; the lowest mixing ratios for tropical upper tropospheric water vapor (blue) occur in the eastern Pacific. In 1997 the sea surface temperatures in the tropical eastern Pacific are much warmer than in 1996, and the region of intense convection shifts eastward away from Indonesia. The situation is reversed in 1997 from 1996 with the highest mixing ratios for upper tropospheric water vapor in the eastern Pacific, and very low mixing ratios over Indonesia. (Images courtesy of William Read, NASA JPL).

Water vapor is the most important greenhouse gas. Some measurements suggest that water vapor is increasing in the stratosphere. This increase may be due to changes in the transport of air between the troposphere and the stratosphere caused by climate change, or it could be due to changes in the microphysical processes within tropical clouds. More measurements of upper tropospheric water vapor, trace gases and particles are needed to untangle the cause-and-effect relationships of these various agents of climate change. We can verify climate models of the atmosphere only with global observations of the atmosphere and its changes over time.

What will Aura Do?

Aura will measure concentrations of greenhouse gases such as methane, water vapor, and ozone in the upper troposphere and lower stratosphere. Aura also will measure both absorbing and reflecting aerosols in the lower stratosphere and lower troposphere and water vapor measurements inside the high tropical clouds, and will make high vertical resolution measurements of some greenhouse gases in a broad swath across the tropical upwelling region. All of these measurements contribute key data for climate modeling and prediction.

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back: What are the processes controlling air quality?




Aura: A mission dedicated to the health of Earth's atmosphere


High Resolution Dynamics Limb Sounder (HIRDLS)

HIRDLS is an infrared limb-scanning radiometer measuring trace gases, temperature, and aerosols in the upper troposphere, stratosphere, and mesosphere. The instrument will provide critical information on atmospheric chemistry and climate. Using vertical and horizontal limb scanning technology, HIRDLS will provide accurate measurements with daily global coverage at high vertical and horizontal resolution.

HIRDLS makes key contributions to each of Aura’s three science questions.

HIRDLS Contributions to Understanding Stratospheric Ozone

The largest ozone depletions occur in the polar winter in the lower stratosphere. HIRDLS will retrieve high vertical resolution daytime and nighttime ozone profiles in this region.

HIRDLS will measure nitrogen dioxide (NO2), nitric acid (HNO3) and CFCs—gases that play a role in stratospheric ozone depletion. Although international agreements have banned their production, CFCs are long-lived and will remain in the stratosphere for several more decades. By measuring profiles of the long-lived gases at 1.2 km vertical resolution, from the upper troposphere into the stratosphere, HIRDLS will make it possible to quantify the transport of air from the troposphere into the stratosphere.

Modeled global map of nitrous oxide The need for high horizontal resolution measurements of the stratosphere is illustrated above. Using a model, the long-lived trace gas N2O is transported by observed winds. The transport processes produce filamentary structures that are predicted but have never been observed globally. HIRDLS high resolution measurements will be able to observe these structures which are signatures of transport.

HIRDLS Contributions to Understanding Air Quality

HIRDLS will measure ozone, nitric acid, and water vapor in the upper troposphere and lower stratosphere. With these measurements, scientists will be able to estimate the amount of stratospheric air that descends into the troposphere and will allow us to separate natural ozone pollution from man-made sources.

HIRDLS Contributions to Understanding Climate Change

HIRDLS will measure water vapor and ozone, both important greenhouse gases. The instrument is also able to distinguish between aerosol types that absorb or reflect incoming solar radiation. HIRDLS will be able to map high thin cirrus clouds that reflect solar radiation.

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Aura: A mission dedicated to the health of Earth's atmosphere


Microwave Limb Sounder (MLS)

MLS is a limb scanning emission microwave radiometer. MLS measures radiation in the GHz and THz frequency ranges (millimeter and submillimeter wavelengths). Aura’s MLS is a major technological advance over the MLS flown on UARS. MLS will measure important ozone-destroying chemical species in the upper troposphere and stratosphere. In addition, MLS has a unique ability to measure trace gases in the presence of ice clouds and volcanic aerosols.

MLS Contributions to Understanding Stratospheric Ozone

Aura’s MLS will continue the ClO and HCl measurements made by UARS. These measurements will inform us about the rate at which stratospheric chlorine is destroying ozone. MLS will also provide the first global measurements of the stratospheric hydroxyl (OH) and hydroperoxy (HO2) radicals that participate in ozone destruction. In addition, MLS will measure bromine monoxide (BrO), a powerful ozone-destroying radical. BrO has both natural and man-made sources.

MLS measurements of ClO and HCl will be especially important in the polar regions. The HCl measurements tell scientists how stable chlorine reservoirs are converted to the ozone destroying radical, ClO. Since the Arctic stratosphere may now be at a threshold for more severe ozone loss, Aura’s MLS data will be especially important.

MLS Contributions to Understanding Air Quality

MLS measures carbon monoxide (CO) and ozone in the upper troposphere. CO is an important trace gas that can indicate where and when their is an exchange of air between the stratosphere and troposphere. CO is also a tropospheric ozone precursor and its appearance in the upper troposphere can indicate strong vertical transport from pollution events.



Global maps of temperature, nitric acid, chlorine monoxide and ozone

UARS MLS simultaneously mapped key chemical constituents nitric acid (HNO3), chlorine monoxide (CLO), and ozone (O3) over the winter polar regions in both Northern (upper) and Southern (lower) Hemispheres where the greatest ozone loss occurs. Aura MLS will map these and other chemicals with better coverage and larger altitude range than UARS MLS. Together with HIRDLS, Aura MLS will measure an array of source, radical and reservoir gases in the active region of the polar stratosphere to give a complete picture of the ozone depletion process and predicted recovery. MLS will also make global measurements of BrO, OH and HO2.

MLS Contributions to Understanding Climate Change

MLS’s measurements of upper tropospheric water vapor, ice content, and temperature will be used to evaluate models and thus reduce the uncertainty in climate forcing. MLS also measures greenhouse gases such as ozone and N2O in the upper troposphere.

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Aura: A mission dedicated to the health of Earth's atmosphere


Ozone Monitoring Instrument (OMI)

OMI is a nadir viewing spectrometer that measures solar reflected and backscattered light in a selected range of the ultraviolet and visible spectrum. The instrument’s 2600-km viewing swath is perpendicular to the orbit track, providing complete daily coverage of the sunlit portion of the atmosphere. OMI is Aura’s primary instrument for tracking global ozone change and will continue the high quality column ozone record begun in 1970 by the Backscatter Ultraviolet Detectror (BUV) onboard the Nimbus-4 satellite. Because OMI has a broader wavelength range and better spectral resolution, OMI will also measure column amounts of trace gases important to ozone chemistry and air quality. OMI will map aerosols and estimate ultraviolet radiation reaching the Earth’s surface. OMI’s horizontal resolution is about four times greater than TOMS'.

The Netherlands Agency for Aerospace Programs (NIVR) and the Finnish Meteorological Institute (FMI) contributed the OMI instrument to the Aura mission. The Netherlands companies, Dutch Space and TNO-TPD, together with Finnish companies, Patria, VTT and SSF, built the instrument. OMI contributes to each of Aura’s three science questions.

OMI Contributions to Understanding Stratospheric Ozone

OMI will continue the 34-year satellite ozone record of SBUV and TOMS, mapping global ozone change. OMI data will support Congressionally mandated and international ozone assessments. Using its broad wavelength range and spectral resolution, OMI scientists will be able to resolve the differences among satellite and ground-based ozone measurements. OMI will also measure the atmospheric column of radicals such as nitrogen dioxide (NO2) and chlorine dioxide (OClO).

OMI Contributions to Understanding Air Quality

Tropospheric ozone, nitrogen dioxide, sulfur dioxide, and aerosols are four of the U. S. Environmental Protection Agency’s six criteria pollutants. OMI will map tropospheric columns of sulfur dioxide and aerosols. OMI measurements will be combined with information from MLS and HIRDLS to produce maps of tropospheric ozone and nitrogen dioxide. OMI will also measure the tropospheric ozone precursor formaldehyde. Scientists will use OMI measurements of ozone and cloud cover to derive the amount of ultraviolet radiation (UV) reaching the Earth’s surface. The National Weather Service will use OMI data to forecast high UV index days for public health awareness.

OMI Contributions to Understanding Climate Change

OMI tracks dust, smoke and industrial aerosols in the troposphere. OMI’s UV measurements allow scientists to distinguish reflecting and absorbing aerosols and thus OMI measurements will help improve climate models.

Tropospheric ozone global map This monthly average map was made by subtracting the stratospheric ozone column from TOMS column ozone. The stratospheric column is calculated using UARS MLS measurements. Higher quality tropospheric ozone maps on a daily basis will be produced from OMI and HIRDLS data. (Image courtesy S. Chandra and J. Ziemke, NASA GSFC).

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Aura: A mission dedicated to the health of Earth's atmosphere


Tropospheric Emission Spectrometer (TES)

TES is an imaging Fourier Transform Spectrometer observing the thermal emission of the Earth’s surface and atmosphere, night and day. TES will measure tropospheric ozone and of other gases important to tropospheric pollution. Satellite tropospheric chemical observations are difficult to make due to the presence of clouds. To overcome this problem TES was designed to observe both downward (in the nadir) and horizontally (across the limb). This observation capability provides measurements of the entire lower atmosphere, from the surface to the stratosphere. NASA’s Jet Propulsion Laboratory provided the TES instrument.

The TES primary objective is to measure trace gases associated with air quality.

TES Contributions to Understanding Stratospheric Ozone

TES limb measurements extend from the Earth’s surface to the middle stratosphere, and the TES spectral range overlaps the spectral range of HIRDLS. As a result, TES’s high resolution spectra will allow scientists to make measurements of some additional stratospheric chemicals as well as improve HIRDLS measurements of chemicals common to both instruments.

TES Contributions to Understanding Air Quality

TES will measure the distribution of gases in the troposphere. TES will provide simultaneous measurements of tropospheric ozone and key gases involved in tropospheric ozone chemistry, such as nitric acid (HNO3) and carbon monoxide (CO). TES data will be used to improve regional ozone pollution models.

TES Contributions to Understanding Climate Change

TES will measure tropospheric water vapor, methane, ozone and aerosols, all of which are relevant to climate change.

Modeled ozone value mixing ration Harvard University's GEOS-CHEM model illustrates the types of global maps of tropospheric ozone that the TES instrument will observe in a single day. This image shows the GEOS-CHEM simulated ozone field at 681 millibars (about 3.1 km in altitude). The white areas are mountainous regions where the surface pressure is below 681 millibars and, therefore, TES ozone data will not be available. (Image courtesy of Daniel Jacob, Harvard University).

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Aura: A mission dedicated to the health of Earth's atmosphere


Mission Synergy: Maximizing Science Results

The Aura instruments were selected and the satellite was designed to maximize science impact. The four Aura instruments have different fields of view and complementary capabilities. The instruments all observe the same air mass within about 13 minutes, a short enough time so the chemical and dynamic changes between observations are small.

Aura puzzle
Chemical and transport processes have led to changes in the stratospheric ozone layer, and scientists need measurements of many different chemical species to puzzle out the causes for these observed changes. Measurements of ozone-destroying radicals such as CLO, NO2, BrO, and OH and reservoir gases such as N2O5, CLONO2, and HNO3 help solve the chemical part of the puzzle. Measurements of long-lived gases such as N2O and CH4 tell scientists about the puzzling effects of transport. Ozone and some other constituents are measured by all the instruments; some constituents like hydrochloric acid are measured by only one. New Aura measurements, such as OH, will help us complete the picture. The new measurements are shown as detached pieces.

Stratospheric Ozone

Understanding stratospheric ozone change involves measurements of both the ozone profile and the total column amount, as well as the chemicals responsible for ozone change.

All of Aura’s instruments make ozone profile measurements. HIRDLS profiles have the highest vertical resolution and extend from cloud tops to the upper stratosphere. MLS measurements have lower vertical resolution than HIRDLS, but MLS can measure ozone in the presence of aerosols and upper tropospheric ice clouds. TES limb ozone measurements overlap the measurements from HIRDLS and MLS up to the middle stratosphere. OMI can also make broad ozone profile measurements using a modified SBUV technique.

To establish the scientific basis for ozone change, scientists must measure the global distribution of the source, reservoir, and radical chemicals in the nitrogen, chlorine, and hydrogen families. Together, the Aura instruments fill this requirement. For example, HIRDLS measures the halocarbons (chlorine source gases) and chlorine nitrate (one of the major chlorine reservoir gases), while MLS measures the radical chlorine monoxide and hydrogen chloride (the other major chlorine reservoir).

Stratospheric aerosols influence ozone concentrations through chemical processes that transform ozone-destroying gases. HIRDLS measures stratospheric aerosols with the best horizontal coverage and highest vertical resolution of Aura’s four instruments. TES provides a backup for HIRDLS. MLS and HIRDLS both measure nitric acid and MLS measures hydrogen chloride (HCl) and chlorine monoxide (ClO), two gases that are transformed by the chemical processes involving aerosols.

Air Quality

Measuring tropospheric ozone and its precursor gases is a major goal for Aura. Aura’s instruments provide two methods of tracking ozone pollution. First, TES measures tropospheric ozone directly. The second estimate of the total tropospheric ozone amount can be obtained by subtracting HIRDLS stratospheric ozone measurements from OMI’s total column ozone measurements. A similar procedure can be used to estimate the tropospheric amount of NO2, an important ozone precursor.

In the clear upper troposphere, Aura instruments provide overlapping measurements of CO (MLS, TES), H2O (MLS, TES, HIRDLS), HNO3 (MLS, TES, HIRDLS). Carbon monoxide (CO) is an ozone precursor and HNO3 is a reservoir gas for NO2. The combination of TES nadir measurements and MLS limb measurements through clouds will provide important new information on the distribution of CO and H2O.

An emerging problem in air quality is the increasing amount of aerosols in the air we breathe. OMI measures aerosols, and distinguishes between smoke, mineral dust, and other aerosols. Both TES and HIRDLS measure aerosol characteristics in the upper troposphere to help scientists understand how aerosols are transported.

Climate Change

Atmospheric chemistry and climate are intimately connected. Ozone, water vapor, and N2O take part in tropospheric chemical processes and are also greenhouse gases. Changes in these and other greenhouse gases can upset the atmosphere’s heat balance and alter climate. Measurements of these gases, their sources and sinks are essential if we are to understand how the climate is changing as a result of human activity. All of the Aura instruments provide information on tropospheric ozone. Clouds and aerosols are also important contributors to climate change. HIRDLS and MLS will measure cirrus clouds. OMI will measure aerosol distributions and cloud distributions and their heights.

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